WG21-P3349R1 Converting Contiguous Iterators To Pointers

No feature-test macro is mentioned.

We can accept PRs to implement and test this change unconditionally. This mostly just limits what users can rely on, but there's an exception as the paper notes:

The wording in this R1 revision requires the library to perform i+n on the original iterator (not only on a pointer to_address(i)), which gives the iterator a chance to check for a bounds overrun.

I believe we're good about this bounds check in debug mode, but I suspect that we don't always conform to this new requirement for implementers in release mode.